Interrupts on the PiFace

I recently acquired both a Raspberry Pi and a PiFace IO board.
I had a rough time finding examples of how to read the input ports via
interrupts (rather than periodically polling for values), especially
for the newer versions of the PiFace python libraries.

After a little research, here’s some simple code that
will print out pin names as you press the input buttons. Button 3
will cause the code to exit: